1. Field of the Invention
The present invention relates to a magneto-optical recording element, especially an erasable magneto-optical recording element, in which the figure of merit is improved and the adhesion to a substrate and various resistance characteristics are improved in a magneto-optical recording medium. Furthermore, the present invention relates to a method for the fabrication of this magneto-optical recording medium.
2. Description of the Prior Art
Recently, research has been vigorously made on high-density recording using a magneto-optical recording medium. According to this recording method, laser beams are projected on a recording medium to locally heat the recording medium and write bits into the recording medium, and the recorded information is read out by utilizing the magneto-optical effect. According to this method, large quantities of informations are recorded at a high density. This magneto-optical recording medium is obtained by forming an amorphous metal vertical magnetization film composed of a rare earth element and a transition metal mainly by sputtering.
In this magneto-optical recording system, improvement of the recording medium per se and formation of a dielectric layer between the substrate and recording medium have been proposed as means for improving the magneto-optical characteristics.
More specifically, in a photomagnetic recording element comprising a transparent substrate, a transparent dielectric layer and a magnetic layer of a magneto-optical recording medium formed on the transparent substrate through the transparent dielectric layer, if the thickness t of the dielectric layer is set so that multiple reflection is caused when laser beams are projected from the substrate side for reproduction, that is, the thickness t of the dielectric layer is set so that the following condition is satisfied: EQU t=.lambda./4n.multidot.(2m+1)
wherein
.lambda. stands for the reproduction wavelength of laser beams, PA1 n stands for the refractive index of the dielectric layer, and PA1 m is a number of 0, 1, 2, 3, . . . ,
an enhancement of a Polar Kerr effect can be obtained and figures of merit can be prominently improved.
As the dielectric material, there can be mentioned oxides such as CeO.sub.2, ZrO.sub.2, TiO.sub.2, Bi.sub.2 O.sub.3 and SiO and non-oxides such as Si.sub.3 N.sub.4, AlN, CdS, SiC and ZnS. Non-oxides are advantageous in that oxygen originating from the dielectric material is not present in the interface of the amorphous metal vertical magnetization film and deterioration of the magnetic layer owing to diffusion of oxygen is obviated, and if a non-oxide dielectric material excellent in the impermeability to water and oxygen in air is selected and used, a dielectric layer which is stable for a long time and has a high reliability can be obtained.
A plastic material has been used for a substrate of an optical disc for high-density recording because the plastic material is light in the weight, cheap in the cost and excellent in the durability and safety and a large quantity of substrates provided with guide tracks can be reproduced by injection molding. Thus, substrates for magneto-optical recording have been prepared by using polymeric material having an excellent light transmittance, such as a polycarbonate resin and a polymethyl methacrylate resin.
Under this background, it is considered that in a magneto-optical recording element comprising a magnetic layer formed on a substrate of this plastic material through a non-oxide type dielectric layer, if reproduction is performed by projecting laser beams from the substrate side, the higher the refractive index of the dielectric layer than that of the substrate, the larger is the enhancement effect.
As the non-oxide type dielectric material having a high refractive index, there can be mentioned ZnS (n=2.35), CdS (n=2.6) and SiC (n&gt;3). However, these dielectric materials are relatively poor in resistance characteristics and if a layer of such a dielectric material is placed in a high-temperature high-humidity environment for a long time, oxygen and water in air are supplied into the magnetic layer through many pinholes formed at the step of preparing this dielectric layer and deterioration phenomena such as oxidation are caused in the magnetic layer. Although Si.sub.3 N.sub.4 has a refractive index of from 1.9 to 2.1, it provides a dense film free of pinholes, which is excellent in various resistance characteristics. Accordingly, it is desired that the refractive index of the Si.sub.3 N.sub.4 dielectric layer will be improved while excellent resistance characteristics are effectively utilized. However, no proposal has been made in the art to realize this desire.